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ENGINEERING  LI^AI 


REPORTS 

ON    THE 

CALAVERAS    DAM    AND    SPILLWAY 

I 

By 
G.  A.  ELLIOTT,  Engineer  of  Spring  Valley  Water  Company 

and  Approved  by 
WM.  MULHOLLAND,  Consulting  Engineer  on  the  Calaveras  Dam 

II 

And  By 
ARTHUR  P.  DAVIS,  Director  of  the  U.  S.  Reclamation  Service 

and 
DANIEL  W.  MEAD,  Consulting  Engineer  of  Madison,  Wisconsin 


SPRING    VALLEY,    WATER    COMPANY 
**  JULY,     1917 


ENGINEERING  LIBRARY 


REPORT   ON    THE 
SPILLWAY  FOR  THE  CALAVERAS  RESERVOIR. 

Abstract  of  Report  of  G.  A.  ELLIOTT,  Engineer  of  Spring  Valley  Water  Company. 


Several  essentials  must  be  kept  in  mind  in  the  location  and  construc- 
tion of  a  spillway.  Its  primary  purpose,  to  protect  the  dam  from  the 
possibility  of  overtopping  during  floods,  renders  it  imperative  to  have 
sufficient  capacity  to  discharge  the  maximum  floods.  The  inlet  and  outlet 
must  be  so  placed  that  no  danger  will  exist  from  the  entering  current 
washing  the  slope  of  the  dam  or  the  discharge  from  cutting  away  the 
heel  of  the  dam.  It  should  be  located  upon  as  firm  a  foundation  as  can 
be  obtained.  Last  but  not  least  in  California,  the  site  must  be  free  from 
earthquake  faults.  The  Calaveras  spillway  was  located  after  an  investi- 
gation carried  on  for  three  years,  during  which  five  sites  were  explored. 
It  is  situated  in  the  sandstone  rock  at  the  west  end  of  the  dam. 

The  drainage  area  directly  tributary  to  Calaveras  reservoir  is  one  hundred 
square  miles.  An  additional  thirty-eight  square  miles  will  be  included 
after  the  construction  of  the  proposed  tunnel  between  the  Upper  Alameda 
and  Honda  streams.  The  average  annual  rainfall  on  this  total  area  is  28 
inches.  The  maximum  precipitation  occurs  at  Mt.  Hamilton,  with  an 
annual  average  of  31.2  inches.  Kecords  of  intensity  of  rainfall  are 
meager.  However,  at  Mt.  Hamilton  a  total  of  9.05  inches  of  rain 
occurred  on  December  21,  1884.  An  old  record  of  rainfall  in  the  vicinity 
of  the  dam  gives  for  March  16,  1899,  a  precipitation  of  3.5  inches  in 
forty  hours,  including  0.7  inches  in  five  hours.  The  largest  flood  of  which 
actual  measurement  was  made  occurred  in  March,  1911,  when  a  maximum 
flow  of  6,840  cubic  feet  per  second,  lasting  for  two  hours,  was  observed. 
This  amounts  to  68.4  second  feet  per  square  mile  of  drainage  area.  In 
view  of  the  data  on  hand  and  with  a  knowledge  of  the  uncertain 
maximum  conditions  of  rainfall  and  runoff  which  happen  in  the  vicinity, 
the  spillway  was  originally  designed  with  a  capacity  of  20,000  cubic  feet 
per  second,  equivalent  to  200  second  feet  per  square  mile.  In  addition 
to  the  flow  capacity  of  the  spillway  channel,  there  is  an  available  storage 
of  9,000  acre  feet  between  the  lip  of  the  spillway  and  the  crest  of 
the  dam. 

The  location  of  the  spillway  is  such  that  in  the  vicinity  of  the  inlet 
end  the  excavation  necessary  is  not  excessive.  As  the  cut  proceeds 
farther  from  the  inlet  the  bank  becomes  higher  and  the  excavation  per 
unit  of  length  increases.  Under  these  conditions  it  was  advisable  in 
the  interest  of  economy  to  decrease  the  width  of  the  cut  as  much  as 
possible.  The  plan  as  finally  worked  out  calls  for  an  inlet  width  of  150 
feet,  15  feet  deep.  The  bottom  of  the  spillway  is  at  elevation  790,  or  15 


feet  below  the  crest  of  the  dam.  From  the  entrance  the  width  of  the 
channel  decreases  gradually,  reaching  a  bottom  width  of  IS1/^  feet  at  a 
point  540  feet  from  the  inlet.  From  this  point  the  section  is  constant  to 
the  outlet,  being  13%  feet  wide  on  the  bottom,  29  feet  wide  at  the  top 
of  the  lined  section  and  15  feet  deep.  The  total  length  of  the  channel 
is  1,100  feet  and  the  elevation  of  the  discharge  end  is  692  feet,  the 
drop  from  inlet  to  outlet  being  98  feet.  Necessarily  the  velocity  of  the 
water  must  increase  in  proportion  to  the  decrease  in  section  of  the 
channel,  so  that  the  narrow  conduit  will  carry  the  same  amount  as  the 
wider  initial  section.  In  order  to  avoid  disturbance  of  the  current,  the 
grade  of  the  bottom  of  the  spillway  was  so  arranged  with  respect  to  its 
width  and  hydraulic  properties  that  the  velocity  of  the  water  is  con- 
tinually increased.  The  upper  or  throat  section  is  540  feet  long  and  it  is 
in  this  section  that  the  incoming  water  is  accelerated  up  to  its  maximum 
velocity  of  66  feet  per  second.  After  this  velocity  is  attained  the  channel 
is  maintained  at  uniform  section  and  grade. 

The  velocity  of  the  water  when  10,000  second  feet  is  passing  through 
the  narrow  section  is  66  feet  per  second.  At  first  thought  this  velocity  may 
appear  to  be  high,  but  there  are  precedents  which  leave  no  doubt  as  to  its 
practicability.  It  is  essential  that  no  obstructions  be  placed  in  the  path 
of  the  stream  and  that  the  sides  and  bottom  of  the  channel  must  be 
smooth  and  so  shaped  as  to  offer  the  minimum  opportunity  for  impact. 
This  has  been  done  in  the  design  under  discussion  by  the  use  of  a  long 
(540-foot)  gradually  converging  throat. 

Many  examples  may  be  cited  of  the  non-injurious  effect  of  water  pass- 
ing over  concrete  at  high  velocities.  Among  them  are  the  LaGrange 
Dam  on  the  Tuolumne  River,  where  for  six  months  of  the  year  water 
with  a  probable  velocity  of  70  feet  per  second  has  passed  with  practically 
no  signs  of  erosion.  This  is  remarkable,  as  the  Tuolumne  River  in  flood 
stages  carries  sand  and  gravel.  The  culvert  through  the  Pathfinder  Dam 
carried  water  in  1899,  from  May  to  October  continuously,  at  velocities 
ranging  from  75  to  90  feet  per  second.  At  the  end  of  this  period  not  only 
was  the  concrete  lining  absolutely  uninjured  but  the  form  marks  were 
still  visible.  This  is  probably  the  most  conclusive  evidence  that  concrete 
is  not  injured  by  erosion  without  impact.  Other  cases  of  concrete  chutes 
used  for  transmitting  water  at  high  velocities  are  found  as  follows: 


Location 

Slope 

Velocity 

Boise   Project 

6%   to   30% 

40   feet   per   second 

Standley  Lake  Project 

9%   to   39% 

Klamath   Project 

20%   to   30% 

46      " 

Belle  Fourche  Wasteway 

4,000   sec.   ft. 

48      " 

Arrowrock   Dam 

100-foot  head 

64      " 

—  2  — 


In  none  of  these  cases  has  there  been  any  trouble  due  to  the  velocity. 
Probably  the  best  summation  of  experience  in  this  connection  is  given 
by  Arthur  P.  Davis,  Esq.,  Chief  Engineer  of  the  U.  S.  Keclamation 
Service,  in  "Engineering  News,"  January  4,  1912,  in  an  article  entitled 
"Safe  Velocities  of  Water  on  Concrete."  He  says  that 

"Where  clear  water  can  be  made  to  glide  over  con- 
crete without  disturbing  its  velocity  or  abruptly 
changing  its  direction,  there  is  no  practical  limit  to 
the  velocities  that  can  be  permitted  without  harm." 

The  exact  effect  of  the  discharge  from  the  spillway  cannot  be  prede- 
termined with  sufficient  accuracy  to  warrant  additional  construction  at 
this  time.  A  transverse  ridge  of  comparatively  hard  sandstone  crossing 
the  line  of  the  spillway  channel  just  before  the  outlet  is  reached,  pre- 
sents an  effective  barrier  against  the  discharged  water  cutting  back  along 
the  line  of  the  channel.  After  the  passage  of  the  floods  of  one  season 
such  action  as  should  be  taken  will  be  apparent  and  necessary  construc- 
tion can  then  be  more  effectively  accomplished. 

Mr.  William  Mulholland,  Chief  Engineer,  Los  Angeles  Water  Depart- 
ment, is  Consulting  Engineer  not  only  on  the  spillway  location  and  design 
but  also  on  the  construction  of  the  dam.  *Mr.  Arthur  P.  Davis,  Director 
of  the  U.  S.  Beclamation  Service,  and. Mr.  D.  W.  Mead,  Consulting  Engi- 
neer, both  of  whom  have  had  long  practical  experience  in  the  design  and 
construction  of  hydraulic  works,  were  called  in  to  review  the  entire 
spillway  plan.  Acting  on  their  recommendation  it  was  decided  to  in- 
crease the  depth  of  the  spillway  from  15  feet  to  18  feet,  thereby  securing 
additional  capacity.  The  joint  report  of  Consulting  Engineers,  Messrs. 
Davis  and  Mead,  is  presented  herewith. 


*  Mr.  Arthur  P.  Davis  was  Chief  Engineer  of  the  U.  S.  Reclamation  Service  for 
eighi  years  prior  to  1915  and  since  that  date  has  been  Director  of  the  Service. 
During  that  time  there  have  been  designed  and  constructed  under  his  supervision, 
collecting  and  distributing  works  sufficient  to  irrigate  almost  two,  million  acres  of 
land.  It  is  not  possible  to  specify  here  in  detail  even  those  features  of  these  irriga- 
tion projects  which  are  well  known.  The  Elephant  Butte  dam  (300  feet  high),  the 
Shoshone  dam  (328  feet  high),  the  Roosevelt  dam  (280  feet  high)  and  the  Arrowrock 
dam  (354  feet  high),  may  be  mentioned  in  passing  as  examples  of  the  extent  of 
the  work  designed  and  executed  under  the  supervision  of  Mr.  Davis.  In  addition 
to  the  Reclamation  Service  work,  Mr;  Davis  was  a  member  of  the  Engineering  Com- 
mission which  went  to  China  in  1914  to  report  on  flood  control  of  the  Huai  River 
Conservancy,  and  has  been  called  in  consultation  by  many  utility  corporations  enga-ged 
in  constructing  hydraulic  works,  such  for  instance  as  the  Pacific  Gas  and  Electric 
Company  in  connection  with  Lake  Spaulding  dam. 

Mr.  Daniel  W.  Mead,  one  of  the  leading  engineers  in  the  United  States,  has  had  a 
large  experience  in  the  design  and  construction  of  water-works  and  hydro-electric 
plants.  He  is  the  author  of  several  standard  engineering  books  and  has  made  a 
special  study  of  hydrology,  a  subject  which  is  important  in  the  design  and  construction 
of  hydraulic  control  works.  Mr.  Mead  was  a  member  of  the  Engineering  Commission 
which  was  sent  to  China  to  study  the  flood  conditions  of  the  Huai  River  Conservancy. 
Recently  he  has  been  Consulting  Engineer  for  the  Miami  Conservancy  Commission, 
created  after  the  Dayton  flood  of  1913,  to  prepare  plans  for  preventing  the  repetition 
of  a  similar  flood. 


REPORT   ON   THE 
CALAVERAS  DAM  AND  SPILLWAY, 

3y  ARTHUR  P.  DAVIS,  Director  of  the  U.  S.  Reclamation  Service,  and 
DANIEL  W.  MEAD,  Consulting  Engineer. 


Mr.  S.  P.  Eastman,  Jul^  6>  1917' 

Vice  President  and  Manager,  Spring  Valley  Water  Company, 

San  Francisco,  California. 
Dear  Sir:— 

The  undersigned  Board  of  Engineers  has  made  an  examination  of  the 
conditions  at  the  Calaveras  Dam,  now  under  construction  for  the  Spring 
Valley  Water  Company  near  Sunol,  California,  with  especial  reference 
to  the  character  and  capacity  of  the  necessary  spillway  provisions.  The 
structure  is  about  75%  completed  and  work  has  proceeded  to  some  extent 
upon  the  excavation  for  a  spillway  which  is  to  be  provided  at  the  west 
end  of  the  dam,  and  it  is  to  this  spillway  structure  that  most  of  our 
attention  and  study  have  been  directed. 

The  dam  itself  is  being  built  mainly  by  the  hydraulic  process,  but  a 
large  amount  of  coarse  material  containing  much  rock  is  deposited  on 
the  outer  edge  in  heavy  retaining  dikes  and  sluiced  material  is  being 
deposited  between.  These  methods  result  in  placing  a  mass  of  tight 
puddle  in  the  body  of  the  dam,  well  calculated  to  resist  the  passage  of 
water,  enclosed  within  bodies  of  coarse  material  adapted  to  resist  the 
action  of  water  and  weather,  and  also  to  resist  any  tendency  to  slide; 
and  assuming  proper  bond  with  foundation,  abutments  and  conduit,  it 
constitutes  a  substantial  and  safe  construction  when  provided  with  an 
adequate  spillway  which  is  essential  to  such  a  structure.  At  the  time  of 
our  visit  to  the  dam  the  construction  was  well  advanced,  and  no 
opportunity  was  possible  for  the  examination  of  the  bond  between  the 
fill  and  the  conduit,  abutments  and  foundation,  but  the  work  in  progress 
which  we  did  see,  was  being  carefully  and  thoroughly  done. 

The  partial  failure  of  the  Necaxa  Dam,  which  occurred  before  com- 
pletion, has  thrown  some  doubt  in  the  minds  of  laymen  upon  the  safety 
of  hydraulic  dam  construction.  We  have  given  careful  consideration  to 
the  causes  of  that  accident  and  to  a  comparison  of  the  conditions,  both 
natural  and  structural,  at  the  Calaveras  Dam. 

According  to  the  best  evidence  obtainable,  the  Necaxa  slide  was 
caused  by  an  insufficient  retaining  dike  at  the  edge  of  the  pond  of  sluiced 
material,  and  the  large  amount  of  water  and  semi-liquid  mud  burst  the 
thin  dike  provided.  No  such  conditions,  nor  any  conditions  approaching 
them,  are  allowed  at  the  Calaveras  Dam.  The  banks  retaining  the  central 
heart  of  sluiced  material  are  heavy,  massive  and  composed  largely  of 


rock  which  has  no  tendency  to  slide  upon  the  slopes  on  which  it  is 
deposited.  The  tests,  carefully  made  by  the  engineers,  show  beyond  any 
doubt  a  progressive  consolidation  of  the  sluiced  material  at  the  outer 
edges  and  the  bottom,  which  is  very  reassuring  as  to  the  generous  margin 
of  safety  against  any  such  accident  either  during  construction  or  after 
the  completion  of  the  dam. 

We  have  carefully  examined  the  abundant  data  on  this  subject  and 
are  perfectly  confident  that  not  the  slightest  danger  of  this  kind  exists 
in  connection  with  this  structure.  The  embankment  is  designed  along 
safe  and  conservative  lines,  but  not  more  so  than  advisable  in  a  structure 
of  this  importance. 

We  have  given  close  attention  to  the  studies  made  upon  the  require- 
ments of  spillway  capacity  and  to  the  plans  for  such  a  spillway.  It  must 
have  a  large  discharge  capacity  and  conduct  water  safely  from  the  top 
of  the  reservoir  to  the  canyon  well  below  the  dam  without  danger  to  the 
structure  and  in  sufficient  quantity  to  provide  beyond  any  doubt  for  the 
maximum  flood  that  can  be  expected  in  the  drainage  area  tributary  to 
this  structure.  The  large  quantities  of  water  to  be  discharged  will  gen- 
erate very  high  velocities,  and  the  provisions  made  in  the  plans  for  such 
velocities  are,  in  our  opinion,  safe  and  reliable  if  the  construction  is 
carried  out  as  planned. 

The  spillway  as  designed  consists  of  a  wide  opening  cut  through  the 
hill  near  the  west  end  of  the  dam,  gradually  narrowing  and  descending 
on  slopes  carefully  calculated  to  accelerate  the  velocity  of  the  water  in  a 
safe  manner,  and  to  carry  it  a  distance  of  about  1,150  feet,  discharging 
about  110  feet  above  the  bottom  of  the  canyon  upon  hard  sloping  rock 
of  the  canyon  wall.  A  spillway  thus  designed  and  built  of  sufficient 
capacity  will  protect  the  dam  from  injury,  but  may  be  expected  to  erode 
the  rock  at  the  point  of  discharge.  Just  what  effect  this  will  have  depends 
upon  the  hardness  and  internal  structure  of  the  rock  and  cannot  accu- 
rately be  foreseen,  but  it  will  be  slow  and  if  any  modification  of  the 
discharge  end  is  required  in  the  future  the  hydrologic  conditions  afford 
abundant  opportunity  for  such  extensions,  repairs  or  alterations  as  may 
be  required  in  the  future.  Some  such  additions  will  doubtless  be  re- 
quired, but  their  character  and  extent  can  be  much  more  accurately 
determined  when  the  needs  develop  than  if  they  were  attempted  now,  and 
can  then  be  more  cheaply  performed. 

The  plans  prepared  by  your  Engineer,  Mr.  G.  A.  Elliott,  show  careful 
consideration  of  the  severe  conditions  to  be  met,  and  we  can  make  no 
suggestions  for  improvement  except  as  to  capacity.  In  location,  shape 
and  general  plan  the  spillway  meets  with  our  unqualified  approval,  and  if 
carefully  constructed  according  to  the  plans  will,  without  any  doubt,  be  a 
safe  structure.  In  our  opinion,  however,  its  capacity  should  be  some- 
what increased.  Although  designed  to  discharge  a  flood  more  than  three 
times  the  magnitude  of  any  yet  observed  at  this  point,  the  disastrous 
effects  of  a  possible  lack  of  capacity  would  be  so  great  that  we  believe 

—  6  — 


a  still  wider  margin  of  safety  should  be  provided.  This  will  somewhat 
increase  the  cost,  but  a  spillway  of  much  larger  capacity  can  be  provided 
at  very  moderate  cost  simply  by  carrying  upward  the  concrete  sides  of 
the  spillway  channel  with  corresponding  additions  for  strength  and 
correspondingly  increasing  the  height  of  the  dam  without  changing,  in 
any  respect,  the  location  or  general  design  of  the  spillway.  The  dam  can 
be  increased  in  height  with  perfect  security  without  increasing  the 
width  of  the  base  or  of  any  part  except  near  the  top,  and  will  therefore 
add  only  a  small  amount  to  the  yardage  and  to  the  resulting  cost. 

The  concrete  lining  of  the  spillway  channel  should  be  built  entirely  in 
the  winter,  and  preferably  during  the  coldest  weather  which  this  climate 
affords,  so  that  a  change  of  temperature  will  place  the  concrete  in  com- 
pression and  have  no  tendency  to  open  any  cracks.  To  the  same  end  it 
would  be  well  to  build  this  section  in  alternate  sections  between  expan- 
sion joints  so  that  one  section  would  be  entirely  set  and  past  the  stage 
of  expansion  due  to  the  increased  temperature  of  the  process  of  setting, 
in  order  that  it  shall  have  undergone  the  contraction  which  follows 
setting  before  the  adjacent  section  is  built.  The  entire  surface  of  the 
spillway  must  be  made  as  smooth  as  possible  and  free  from  cracks  or 
projections  of  any  kind,  so  as  to  give  the  water  the  freest  and  smoothest 
possible  discharge  and  avoid  any  chance  for  the  water  to  take  hold  of  the 
concrete.  The  central  line  of  the  spillway  channel  rs  entirely  free  from 
any  horizontal  curvature  and  the  vertical  curvature  is  very  slight,  being 
only  that  required  for  hydraulic  reasons  to  control  the  velocity,  and  if 
constructed  as  planned  will  meet  the  requirement  that  "where  clear 
water  can  be  made  to  glide  over  concrete  without  disturbing  its  velocity 
or  abruptly  changing  its  direction,  there  is  no  practical  limit  to  the 
velocities  that  can  be  permitted  without  harm."  The  above  rule  has 
been  much  discussed  and  tested  by  many  engineers  under  a  large  variety 
of  circumstances,  and  no  exceptions  to  its  validity  have  been  found. 

The  maximum  flood  flow  that  should  be  controlled  by  the  proposed 
spillway  to  be  built  for  the  Calaveras  Dam  may  be  estimated  both  on 
the  basis  of  runoff  of  similar  streams  and  on  the  basis  of  the  runoff 
which  may  result  from  the  extreme  rainfall  that  may  occur  on  the 
drainage  area  above  the  dam.  The  records  of  the  actual  runoff  and 
rainfall  that  have  occurred  in  the  past  on  the  Calaveras  drainage  area 
do  not  cover  a  sufficient  period  of  years  to  indicate  the  extremes  that 
are  liable  to  occur  in  the  lapse  of  time. 

Many  illustrations  might  be  given  to  show  that  extreme  local  rainfall 
or  runoff  may  occur  only  at  long  intervals  of  time,  perhaps  one  hundred 
to  two  hundred  years  apart,  and  that  the  extremes  for  a  thousand  years 
or  more  do  not  greatly  exceed  the  extremes  for  one  hundred  to  two 
hundred  years. 

It  seems  apparent,  therefore,  that  the  only  safe  criterion  for  either 
flood  flow  or  rainfall  is  the  extremes  that  have  occurred  on  areas  more  or 
less  similar  and  over  a  sufficient  range  of  time  and  geographic  extent 
to  make  it  certain  that  the  extremes  have  been  covered,  or  failing 


RECORDED  FLOODS  IN   CALIFORNIA 

Stream 

Area 
Drained 

Sec.  Ft.  per 
Sq.   Mile 

Calaveras  River  

395 

176 

San    Gabriel 

220 

215 

Arroyo    Seco  

38.6 

366 

Arroyo    Seco 

30.5 

374 

Verdusro    Creek.... 

21.9 

352 

such  sufficient   data  to   allow  a  sufficient  factor  of  safety  to  eliminate 
the  risk  involved  in  the  problem  at  hand. 

The  following  floods  have  been  observed  in  California: 


Date 
1911 
1884 
1914 
1914 
1914 

The  rainfalls  which  produced  the  above  floods  have  been  so  far 
exceeded  in  various  parts  of  California  that  we  do  not  consider  them  the 
most  extreme  conditions  possible.  In  fact,  judging  from  rainfall  records 
and  flood  damages,  it  seems  probable  that  still  heavier  floods  were 
discharged  in  1916. 

Actual  flood  data  are,  however,  so  meager  that  we  must  consider  also 
the  far  more  abundant  data  of  rainfall. 

In  considering  flood  flows  based  on  rainfall,  it  must  be  remembered 
that  the  peak  flood  in  a  day  may  be  much  more  intense  than  the  rainfall 
that  creates  it,  and  that  the  runoff  in  24  hours  may  be  greater  than  the 
maximum  24-hour  rainfall  of  a  given  storm  when  the  storm  lasts  for 
several  days. 

A  few  rainfall  records  in  California  extend  back  about  66  years.  The 
official  records  of  the  U.  S.  Signal  Service  began  in  1871.  The  work  was 
taken  over  by  the  Weather  Bureau  in  1896,  since  which  date  the  number 
of  stations  has  been  gradually  increased.  It  is  evident  that  the  records 
available  are  brief  and  incomplete,  and  it  is  hardly  possible  that  they 
in  any  place  show  either  the  maximum  or  the  minimum  that  must  be 
expected  in  the  course  of  time  and  which  may  occur  at  any  time  when 
all  the  factors  happen  to  become  favorable  to  extreme  conditions. 

When  the  number  and  location  of  observations  in  the  United  States 
and  in  California  are  considered  in  relation  to  the  broad  areas  and 
especially  the  mountain  areas,  for  which  few  observations  have  been 
taken,  it  is  quite  evident  that  even  the  extremes  shown  in  the  records 
available  are  in  all  probability  not  the  maximum  for  the  United  States 
or  for  the  State. 

The  State  of  California  is  particularly  subject  to  extreme  rainfall 
conditions  on  account  of: 

1.  The  prevailing  easterly  drift  of  the  surface  atmos- 

phere. 

2.  The  proximity  of  the  Pacific  Ocean;    and 

3.  The  diversified  topography  of  the  State. 

The  average  annual  rainfall  varies  from  100.14  at 
Helen  Mine  (elevation  2,750)  to  2  at  Stirling; 
with  annual  extremes  of  from  153.54  at  Monu- 
mental to  a  Trace  at  Salton. 

—  8  — 


The  annual  rainfall  in  California  therefore  varies  practically  frtom  the 
maximum  to  the  minimum  of  that  which  has  been  observed  within  the 
United  States.  In  intensity  of  rainfall,  the  records  of  California  contain 
extremes  closely  approximating  the  maximum  observed  in  the  United 
States. 

On  August  12,  1891,  at  Campo,  San  Diego  County  (Elevation  2,543) 
11.5  inches  of  rain  fell  in  80  minutes  and  the  total  storm  amounted  to 
16.1  inches.  Other  records  of  rainfalls  from  one  to  five  days'  duration 
are  as  follows: 


Station                     Elev. 
Lick      Observatory   4209 

Magalia    2320 
Mono   Ranch  3210 
Helen  Mine  2750 
Inskip    4975 
West   Branch              3216 

Average 
Annual 

32.28 
83.12 

100.14 

100  ? 
91  ? 

Date 
Dec.     1884 
Jan.      1911 
Jan.      1906 
Mar.     1906 
Dec.     1913 
Dec.     1913 
Dec.     1910 

9.05* 
9.19* 
10.86 
11.50 
10.40 
10.35 
10.00 

Inches 

14.75 
13.23 
13.60 

in   1   to 

15.47 
14.23 
14.25 

5  Days  —  ^ 

Nellie     

5350 

44  '? 

Jan. 

1916 

11.24 

17.24 

20.76 

22.61 

23.12 

Jan. 

1916 

10.16 

12.43 

13.40 

15 

.25 

16 

.10 

Rialto     

....   2250 

69  ? 

Jan. 

1916 

12.95 

15.99 

18.79 

20 

.16 

20 

.27 

Squirrel   Inn   .... 

....   5280 

37.22 

Jan. 

1916 

16.81 

22.64 

25.66 

26 

.87 

27 

.82 

Los   Gatos   

....      600 

33.33 

Jan. 

1911 

6.15 

9.95 

13.30 

16 

.15 

16 

.31 

San  Leandro 

48 

23.34 

Jan. 

1911 

4.70 

5.76 

7.41 

8 

.31 

8 

.42 

Livermore    ..   .. 

483 

15.54 

Jan. 

1911 

2.54 

4.33 

6.83 

7 

.53 

7 

.68 

Needles    .. 

427 

3.52 

Julv 

1914 

3.75 

The  only  records  in  the  United  States  exceeding  those  given  above 
are  the  following: 

Alexandria,   La  .................................  June   15-16  1886  21.4  inches  in   24  Hrs. 

Altapass,    N.   C  ...................................  July    15-16  1916  22.22  "  "    24 

Montell,     Texas  ..................................  Jan.    28-29  1913  20.60  "  "    18%  " 

Concord,    Pa  .......................................  Aug.      5  1843  16.  "  "3  " 

Guinea,    Va  .....................................  ....  Aug.    24  1906  9.30  "  "      1  " 

Galveston,     Texas  ..............................  June     4  1871  3.94  "  "14  " 

Ft.    McPherson,    Neb  .........................  May    27  1868  1.50  "  "5 

As  a  general  law  (which  is  substantiated  by  conditions  in  California) 
most  intense  rainfall  is  induced  when  moist  winds  are  dynamically  cooled 
on  mountain  slopes  adjacent  to  the  sea,  and  the  lower  lands  and  the 
interior  mountains,  unless  of  greater  heights  than  the  first  range  en- 
countered, receive  less  average  rainfall  than  the  ocean  slopes  of  the 
first  range.  Hence  stations  on  the  Western  slope  of  the  Coast  Range 
and  the  high  western  slopes  of  the  Sierras  receive  the  maximum  average 
and  maximum  annual  rainfalls. 

As  shown  by  a  great  number  of  records  in  California,  rainfalls  of  5 
inches  and  more  usually  occur  on  the  western  slopes  of  the  mountains 
and  at  altitudes  of  1000  feet  or  more;  yet  at  Los  Gatos  (elevation  600) 
on  the  eastern  side  of  the  outer  Coast  Range,  a  rainfall  of  6.15  inches 


*  Precipitation  probably  occurred  in  two  or  more  days. 
?  Average    annual    rainfall    approximate. 


occurred  in  one  day  in  January,  1916.  While  so  far  as  the  records  go  no 
rainfall  of  10  inches  per  day,  or  more,  has  occurred  at  elevations  of  less 
than  2200  feet,  it  is  evident  from  the  Alexandria,  La.,  record  that  such 
an  occurrence  is  not  impossible. 

The  drainage  area  above  the  Calaveras  Dam,  which  is  long  and  narrow, 
varies  in  height  from  790  feet  at  the  reservoir  to  4209  at  Mt.  Hamilton. 
The  average  annual  rainfall  on  the  area  seems  to  be  about  28.5  inches, 
and  few  records  of  intense  rainfalls  are  obtainable  even  at  Lick  Observa- 
tory, on  Mt.  Hamilton,  where  observations  have  been  made  since  January, 
1881.  The  Lick  Observatory  rainfall  records  are,  however,  somewhat 
unsatisfactory,  inasmuch  as  the  records  available  do  not  show  intensity 
and  frequently  cover  only  the  total  rainfall  for  a  number  of  days.  For 
example,  the  extensive  rainfalls  of  9.05  and  9.19  inches  shown  in  the 
preceding  table  are  believed  to  have  fallen  in  two  or  more  days  instead  of 
in  24  hours.  The  Lick  Observatory  records  for  December,  1884,  show  a 
total  of  33.84  inches;  but  this  is  believed  by  Professor  A.  G.  McAdie  to 
be  a  doubtful  record,  though  if  true  it  probably  represents  some  very  high 
24-hour  rainfalls  during  that  month.  It  should  be  noted  that  at  Liver- 
more,  which  has  an  average  annual  rainfall  of  15.54  inches,  and  which  is 
better  protected  from  extreme  conditions  than  the  Calaveras  drainage 
area,  a  rainfall  of  7.68  inches,  which  is  about  half  of  the  average  annual, 
occurred  in  five  days.  The  Los  Gatos  record  of  January,  1911,  of  6.15 
inches  in  one  day  and  16.15  inches  in  four  days,  might  be  equalled  or  even 
exceeded  on  the  Calaveras  drainage  area. 

January  18-21,  1914,  and  January  12-19,  1916,  rainfalls  of  from  8  to  12 
inches  or  more  occurred  over  areas  near  Los  Angeles,  greater  in  extent 
and  of  similar  elevation  to  the  Calaveras  drainage  area,  and  on  the  latter 
date  a  similar  rainfall  extended  over  similar  areas  in  the  neighborhood 
of  San  Diego,  and  apparently  other  similar  excessive  rainfalls  have 
occurred  in  many  other  portions  of  the  State. 

It  seems  apparent  that  on  account  of  the  southern  location  more 
extreme  rainfall  conditions  are  liable  to  occur  more  frequently  in  South- 
ern California,  near  the  coast  in  Central  California,  and  on  the  high 
Sierras  than  on  the  low  mountains  behind  the  first  mountains  of  the 
Coast  Eange;  but  it  seems  within  possibility,  although  unlikely,  that  an 
average  rainfall  of  from  8  to  16  inches  or  more  might  occur  on  the 
Calaveras  drainage  area  which  might  result  in  a  runoff  of  12  inches  in  24 
hours  or  an  average  of  32,000  second  feet,  with  a  peak  of  perhaps  44,000 
second  feet. 

From  the  above,  and  other  data  not  discussed  in  detail,  the  conclusion 
is  drawn  that  the  Calaveras  Dam  spillway,  with  the  reservoir  storage 
available  above  elevation  790,  should  provide  for  a  peak  flood  of  about 
44,000  second  feet,  an  average  24-hour  flood  of  about  32,000  second  feet, 
and  a  maximum  discharge  of  about  26,500  second  feet.  While  it  is  true 
that  this  will  give  a  capacity  about  seven  times  that  which  would  have 
been  needed  for  the  flood  of  March,  1911,  it  is  believed  to  be  warranted 

-10- 


by  the  serious  results  that  would  follow  a  failure  to  carry  safely  the 
extreme  flood. 

In  view  of  the  above  it  is  our  opinion  that  it  would  be  well  to  design 
the  slopes  of  the  spillway  with  a  view  to  the  smooth  discharge  of  20,000 
second  feet,  and  the  height  and  strength  of  its  walls  for  a  maximum 
discharge  of  26,500  second  feet. 

The  increase  of  the  drainage  area  on  account  of  the  construction  of 
the  Upper  Alameda  tunnel  need  not  be  considered,  as  this  tunnel  will  not 
be  large  enough  to  sensibly  affect  the  volume  of  a  great  flood. 

The  question  has  been  raised  whether  it  is  desirable  to  provide  perma- 
nent gates  to  the  large  concrete  conduit  which  has  been  used  during 
construction  to  convey  the  waters  of  the  creek  past  the  dam  site  so  as 
not  to  interfere  with  construction,  in  addition  to  the  regular  discharge 
gates  in  the  inlet  tower.  This  conduit  will  continue  in  use  to  discharge 
the  water  admitted  to  the  tower  from  the  reservoir  when  required  for  use, 
but  beyond  this  we  believe  it  is  not  only  unnecessary  but  undesirable  to 
install  any  device  for  drawing  large  quantities  of  water  from  the  bottom 
of  the  reservoir  for  other  purposes,  or  to  maintain  the  temporary  gates 
already  installed,  which  are  manifestly  unsuitable  for  permanent  use. 
They  would  serve  no  useful  purpose  after  the  dam  is  completed,  but  we 
advise  that  they  be  not  permanently  closed  until  that  time. 

Eespectfully  yours, 

AKTHUE  P.   DAVIS, 
DANIEL    W.    MEAD. 


—  11  — 


photomount 

pamphlet 

Binder 

Gaylord  Bros 
Makers 
Stockton 

PUT.  »H.  21, 


Inc. 


B27M67TC 


LJb 


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